This invention in general relates to dielectric waveguides, and in particular to a method for fabricating glass preforms from which optical communications fibers can be drawn.
The use of optical fiber as a medium for transmitting high volumes of information over large distances is now a well-established practice known throughout the communications industry. In fact, the performance of optical fibers has improved so steadily since the possibility of their use for communications purposes became evident in 1966 that some believe that they could displace conventional copper wire based systems as early as the turn of the century. This rapid progress has been due largely to developments in materials identification and in methods for fabricating fiber.
Optical fibers are thin filaments of glass having a central region of high index of refraction, or high effective index of refraction where the index varies, surrounded by a sheath or cladding region of lower index, a structure which causes optical radiation properly injected into the fiber end to propagate along the length of the fiber and emerge from the distant end.
The performance of optical fibers for communications is chiefly determined by optical loss or attenuation and by dispersion. Losses are caused by absorption, scattering, and imperfect geometry or structural defects and dispersion, which causes a smearing of light pulses leading to noise, is of two types. One type of dispersion is a change in refractive index with wavelength due to the material itself, and the other is referred to as modal dispersion which is due to differences in the optical path lengths for different transmission modes.
For optical fibers to be competitive with copper wire based systems in transmitting high data rates over long distances, they must have low transmission losses and produce but low signal distortion. In order to obtain such high quality optical fibers, extremely pure glasses are required since even traces of certain impurities such as Fe or Cu increase the attenuation drastically. And, in order to have wide band transmission, it is necessary to carefully control the refractive index profile and the material dispersion in the fiber since it determines in large part the distortion of the signals transmitted along the fiber.
To meet these requirements, those skilled in the art have developed various techniques by which optical fibers can be successfully fabricated. These techniques are, by and large, based upon the formation of silica based glass from appropriate glass precursor vapors. Techniques used have been the soot process (U.S. Pat. No. 3,711,262 and U.S. Pat. No. Re. 28,029); the modified chemical vapor deposition process (U.S. Pat. No. 4,217,027); and the vapor axial deposition process (U.S. Pat. Nos. 3,966,446; 4,135,901 and 4,224,046). As a result of these processes and improvements to them, optical fibers are now routinely fabricated in commercial processes with losses less than two db/km in certain parts of the optical region of the spectrum.
One example of a significant improvement over processes which relied on the external deposition of soots from vapor phase precursors to form preforms from which fibers are drawn was to reduce the inherent hydroxyl ion (OH) content to levels sufficiently low not to present a problem of undue absorption in the wavelength regions of interest. Prior to this improvement, it was known that a successful fiber of low attenuation required that the hydroxyl ion content had to be below a few parts per million because of the existance of undesirable OH absorption at overtones of the fundamental stretching vibration of OH which centered around 2.8 microns. These overtones give rise to absorptions at 1.4 microns and 970 and 750 nm and thus interfere with a transmission band of interest in glass. Thus, the OH ion which was ever present in such processes had to be precluded from the final fiber if low transmission losses were to be achieved. The elimination of the OH ion was a particularly vexing problem for the industry because of its presence in undesirable quantities in most of the vapor phase processes. To solve this problem, chlorine has been used as a drying agent to remove the OH ion from preforms made from flame hydrolysis as shown and described in U.S. Pat. No. 3,933,454. Fluorine has also been proposed as a drying agent as shown and described in U.S. Pat. No. 4,065,280 and as a dopant in a fiber for purposes of reducing hydroxyl ion content as shown and described, for example, in U.S. Pat. No. 4,441,788. In addition, fluorine has been used to reduce cladding index (U.S. Pat. No. 4,082,420).
To improve transmission bandwidth, those skilled in the art chose to use single mode fibers rather than multimode fibers because the use of single mode fibers eliminated or virtually eliminated dispersion manifested as pulse spreading due to differences in optical path length between the various modes propagating in a multimode fiber and to material dispersion as well.
Initial fabrication of single mode fibers was of the step index type in which the core was of uniform index of refraction and the cladding was primarily of a uniform lower index of refraction. Early fibers comprised silica cores with doped claddings of, for example, borosilicate and later fluorosilicate. Later fibers included undoped cores of, for example, germania silicate and silica claddings. However, these designs presented manufacturing problems because of the high temperatures necessary to process deposited pure silica.
The later fibers included germania silicate cores and phosphosilicate claddings. Phosphorus in the cladding simplifies the manufacturing process because it lowers the melting temperature of silica. Furthermore, the removal of boron from the fiber, whose presence likewise simplifies manufacturing due to lowered melting temperatures, avoids the relatively low wavelength infrared absorption edge associated with borosilicate glasses.
In spite of the many innovations made in this art, improved fiber material structures and manufacturing processes are still needed to assure low loss and low dispersion which, in turn, translate into long distances between repeaters and to high telecommunications capacity. Accordingly, it is a primary object of the present invention to provide a method for fabricating preforms from which single mode optical fiber of low attenuation and dispersion and favorable characteristics for fabrication can be drawn.
It is another object of the present invention to provide an efficient method by which high volumes of fiber can be drawn.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. The invention accordingly comprises the steps exemplified in the detailed disclosure which follows.